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<head>
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  <title>Document</title>
  <style>
        *{
      padding: 0;
      margin: 0;
    }
    a{
      color: inherit;
      text-decoration: none;
    }
    .menu{
      margin-left: 50px;
      width: 150px;
      text-align: center;
      background-color: #eee;
    }
    .menu a{
      display: block;
      padding: 10px;
    }
    ul{
      list-style: none;
    }
    .dropdown{
      position: relative;
    }
    .dropdown > ul{
      position: absolute;
      left: 100%;
      top: 0;
      width: 100%;
      background-color: #eee;
      display: none;
    }
    .dropdown:hover > ul{
      display: block;
    }
    .dropdown a:hover{
      background-color: #ccc;
    }
    .dropdown:hover{
      background-color: #ccc;
    }
    .dropdown::after{
      content: '▼';
      position: absolute;
      right: 15px;
      top: 11px;
    }
    .dropdown:hover::after{
      content: '▶';
    }
  </style>
</head>
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  <ul class="menu">
    <li class="dropdown">
      <a href="#">关于我们</a>
      <ul >
          <li class="dropdown">
            <a href="#">子菜单1-1</a>
            <ul>
              <li><a href="#">子菜单1-1-1</a></li>
              <li><a href="#">子菜单1-1-2</a></li>
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            <a href="#">子菜单1-2</a>
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              <li><a href="#">子菜单1-2-3</a></li>
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          <li><a href="#">子菜单1-3</a></li>
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</body>
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